A need exists to provide better organic light emitting devices (OLEDs) including better materials used in the devices. In particular, better OLED devices and materials, including hole transporting materials (HTMs), are needed. An OLED operation is based on injection, transport and recombination of two types of charge carriers: holes and electrons. It is important in an OLED device to control the injection and transport of these two types of carriers so as to enable the recombination to occur in the EML where the luminescent species are located. The location where these species meet and recombine can dictate the efficiency and lifetime of the device. Vapor processed OLED devices can adopt a multilayer strategy of using complex device architecture of 6-8 layers at times to effectively control and alter the charge carrier flow as needed to optimize performance. However, for solution processed devices, it can be more challenging to form multilayer structures as the solvent for a given layer can re-dissolve the previously applied layer. The vapor approach gives good performance and has seen some adoption in the industry, but solution processing holds the promise of significantly higher throughput and lower costs and, because of that, is of great commercial promise.
In particular, a need exists for a good platform system to control properties of hole injection and transport layers such as solubility, thermal stability, and electronic energy levels such as HOMO and LUMO, so that the materials can be adapted for different applications and to function with different materials such as light emitting layers, photoactive layers, and electrodes. In particular, good solubility and intractability properties are important. The ability to formulate the system for a particular application and provide the required balance of properties are also important.
Additional background material can be found in, for example, (a) Charge carrier transporting molecular materials and their applications in devices, Shirota, et al., Chem. Rev., 2007, 107, 953-1010, (b) Organic electroluminescent diodes, Tang, et al., Appl. Phys. Lett. 1987, 51, 913-915.